The Invisible Map of Bird Migration
Each autumn, billions of birds leave their breeding grounds and travel, often at night and frequently over open sea, to places they may never have seen before. A small songbird weighing little more than a few coins might cover several thousand kilometres and then, the following spring, return to the same patch of woodland in which it hatched. For centuries, observers could only marvel at this. Even now, after decades of careful experimentation, it would be an exaggeration to claim that the problem of how birds find their way has been solved. What has changed is that we understand why it proved so difficult: there is not one answer but several, and birds appear to combine them in ways that researchers are only beginning to disentangle.
The first systematic clues came from caged birds. Migratory species held in captivity become restless at the time of year when they would normally depart, hopping and fluttering persistently in one direction. By placing such birds in funnel-shaped cages lined with paper that recorded their footprints, investigators could measure which way they were trying to go. When the cages were taken into a planetarium and the projected night sky was rotated, the birds changed direction accordingly, demonstrating that they could orient themselves by the stars. Other experiments, in which mirrors were used to shift the apparent position of the sun, showed that daytime migrants used the sun as a compass, compensating for its movement across the sky with an internal clock.
Yet the sun and stars cannot explain everything, since birds continue to migrate successfully under thick cloud. Here the evidence points to an ability to sense the earth's magnetic field. Birds exposed to artificially altered magnetic fields shift their orientation in predictable ways, and the mechanism, although still debated, may involve chemical reactions in the eye that are influenced by magnetism. If this is correct, a bird might literally see the field as a faint pattern laid over its normal vision. It is an extraordinary proposal, and I confess that when I first encountered it I found it hard to take seriously. The accumulated evidence, however, is now difficult to dismiss.
A compass, though, only tells an animal which way it is facing; it does not tell it where it is. To return to a particular wood, a bird also needs something resembling a map, and here the picture becomes more contested. Some seabirds seem to rely heavily on smell, building up a picture of the ocean from the odours carried on the wind; individuals whose sense of smell has been temporarily blocked often fail to find their way home from unfamiliar release sites. Other species may detect gradual variations in the strength of the magnetic field across the globe, and familiar landmarks such as coastlines and mountain ranges undoubtedly matter in the final stages of a journey. The danger, in my view, is that researchers working on a single cue have sometimes written as though it alone were sufficient, when the more plausible explanation is that birds switch between sources of information depending on conditions.
Perhaps the most striking findings concern young birds on their first migration. In many species the young travel separately from their parents, often departing weeks later, and yet they set off in broadly the right direction. Experiments in which young birds were moved hundreds of kilometres before departure have shown that they continue on their original heading and consequently end up in the wrong place, whereas experienced adults correct for the displacement. This suggests that first-time migrants inherit a direction and a rough duration of travel, but that the ability to reach a precise destination is learned. A clock-and-compass programme, in other words, is enough to bring a young bird into approximately the right region, after which experience refines the route.
These are not merely academic questions. Many migratory species are in decline, and understanding how they navigate helps to explain why. Artificial light, for instance, can draw night-flying birds off course, causing them to circle brightly lit buildings until they are exhausted. Some cities now dim the lights of tall buildings during peak migration periods, a measure that seems to me both inexpensive and long overdue. Equally important are the stopover sites where birds rest and feed, sometimes almost doubling their body weight before crossing a desert or a sea. When a single wetland on which large numbers depend is drained, the consequences may be felt thousands of kilometres away.
Climate change adds a further complication. Because many species time their departure by day length, which has not changed, rather than by temperature, which has, they may arrive at their breeding grounds after the insects on which their chicks depend have already passed their peak. Some populations do seem to be adjusting, leaving earlier or shortening their journeys. Whether such adjustments can keep pace with the speed of environmental change is uncertain, and it would be unwise to assume that a system refined over millions of years will adapt to new conditions within a few decades.
